The FCC has recently made the 902-928 MHz band and two higher frequency bands available under Part 15 rules for low-power communications devices with the proviso that spread-spectrum (SS) modulation be used. One SS technique permitted is frequency hopping in which information is sent using a sequence of carrier frequencies that change at set times to produce a narrow band signal that bounces around in center frequency over the available spectrum. The FCC specified that at least 75 hopping frequencies, separated by at least 25 kHz, must be used for each communication channel and the average time of occupancy on any frequency must not be greater than 0.4 seconds within a 30-second period.
Since the 902-928 MHz band contains only 1,040 frequency bands of 25 kHz each, only six simultaneous, independent, two-way links (2 channels) could be supported without interference among the channels. This is because each channel requires at least 75 of the 25 kHz frequency bands for its frequency hopping transmission. A problem arises in applications where a communication system requires more than six communication links in a small geographic area. While directional antennas may be utilized to isolate groups of stations using the same frequencies, interference may still result.